1. Field of the Invention
This invention relates to a process for forming conjugated conductive polymer films or coatings on a substrate. Another aspect of this invention relates to composites comprising a conjugated conductive polymer film or coating on a substrate.
2. Prior Art
Polyaniline is a conducting polymer which is of considerable interest for a variety of applications because of high thermal and environmental stability, high electrochemical recyclability, special optical properties, and very high conductivity compared with conventional polymers. Many of these applications utilize polyaniline as thin film coatings, such as in electrochromic windows and displays, antistat and electrostatic charge dissipation layers, memory layers, rechargeable batteries, sensors, and separation membranes. Examples of such applications of polyaniline are described in (1) "Electrochromism of Polyaniline Film Prepared by Electrochemical Polymerization" by A. Watanabe, K. Mori, Y. Iwasaki, Y Nakamura, and S. Niizuma, Macromolecules, 20, 1973-1796 (1987), (2) "Adjustable Tint Window With Electroactive Conductive Polymers" by J. F. Wolf, L. W. Shacklette, G. G. Miller, R. L. Elsenbaumer and R. H. Baughman in U.S. Pat. No. 4,893,908, (3) "Electrochemistry Employing Polyaniline" by A. G. MacDrarmid and N. L. D. Somasiri in U.S. Pat. No. 4,820,595, (4) "Polyaniline: A Historial Survey", by E. M. Genies, A. Boyle, M. Lapkowski, and C. Tsintavis in Synthetic Metals, 36, 139-182 (1990), (5) "Conductive Thin Films and Membranes" by S. K. Bahador in Makromol. Chem., Makromol Symp. 37, 129-147 (1990), (6) "Chromatic Changes in Polyaniline Films" by C. D. Batich, H. A. Laitinen and H. C. Zhou, J. Electrochem. Soc. 137, 883-885 (1990), and (7) "Memory Effects Conducting Polymers" by B. Villeret and M. Nechtschein in Phys. Rev. Lett., 63, 1285, (1989) and (8) "Fabrication of Printed Circuit Boards Using Conducting Polymer" by H. W. Song, J. M. Park and J. R. White in European Patent Application 0413109A2.
Various methods for forming polyaniline films or film coatings are described in the literature. The above references 1-7 describe the electrochemical polymerization of polyaniline. This method requires a conductive substrate, so it is unsuitable for the coating of conventional plastics, and is general expensive. As an alternative to coating by electrochemical processes, polyaniline can be chemically synthesized (see above ref. 4) and then deposited as a film coating on a substrate by either solution methods (see "Polyaniline Solutions, Films and Oxidation State" by M. Angelopoulos, G. E. Asturias, S. P. Ermer, A. Ray, E. M. Scherr, A. G. MacDiarmid, M. Akhtar, F. Kiss and A. J. Epstein Mol. Cryst. Lig. Cryst., 160, 151-163 (1988) and the above reference 4) or by use of polymer latices (see "Electrically Conducting Organic Films and Beads Based on Conducting Latex Partices" by E. C. Cooper and B. Vincent J. Phys. D: Appl. Phys. 22, 1580-1585 (1989)). Each of these methods has the disadvantage of requiring separate polymer synthesis and film coating steps. Also, except by using strong acids as solvent or special dopant ions, conductive polyaniline cannot be directly formed from solution so that post deposition doping or thermal processing is usually required to generate the conducting polymer. Additionally, sublimation processes result in low molecular weight coatings and forming adhesive coatings is typically a problem for each of these processes. While improvements in the solubility characteristics of conductive polyaniline can be obtained by appropriate ring or nitrogen substitutions or the use of polymeric counterions, such improvements typically result in a decrease of obtainable conductivity (see "Water-Soluble Conducting Polyaniline Polymer" by J-Y Bergerun, J-W. Chevalier, and L. H. Dao, J. Chem. Soc., Chem. Commun, 1990, 180-182, and "Conductive Polymer Materials and Methods and Methods of Producing Same" by S. I. Yaniger and R. E. Cameron in U.S. Pat. No. 4,851,487).
In contrast with the above methods, which require separate polymerization and film forming steps, it is also possible to directly polymerize aniline as a coating material. For example, H. H. Kuhn describes in European Application 0 352 882 A1 the preparation of conductive fabrics by contacting the fiber under agitation conditions with an aqueous solution of an aniline compound, oxidizing agent, and a doping agent or counter ion, the oxidizing agent being a vanadyl compound. Also, C.-H Hsu and E. Vance describe in European Application 0355 518 A2 the process of forming a conductive coating by imbibing an aqueous aniline solution on a organic fiber or film and polymerizing the aniline in situ by contacting the fiber or film with an oxidizing catalyst in an acid aqueous medium. Improvements are needed which provide highly adhering, highly conducting polymers such as polyaniline via a rapid, cost effective process which simultaneously provides polymerization and coating formation.